W. C. Chen

Solubility and kinetic properties of deuterium in single crystal Pd

Abstract The pressure–composition (PC) behavior of deuterium in single crystal Pd has been measured over a temperature range extending from room temperature to 393 K. These data allow the determination of (i) relative partial molar enthalpy and entropy, without the contribution of the configuration entropy, for the dilute solid solution phase, (ii) enthalpy and entropy change characterizing the α→α′ phase transformation, and (iii) Gibbs free energy loss associated with the PC thermodynamic hysteresis. Comparison of these quantities with published data for polycrystal Pd and with our own polycrystal Pd measurements show good agreement, indicating grain boundary interfaces and second phase inclusions known to exist in polycrystalline Pd do not influence these thermodynamic properties. The time dependence of deuterium absorption and desorption from the gas phase has been measured in both single and polycrystal Pd as well. The activation energy and diffusion constant of deuterium in the solid solution phase of Pd agree with published values. The time dependence of deuterium absorption and desorption within the miscibility gap are reduced by approximately two orders of magnitude compared to solid solution behavior. The two-phase kinetic data exhibit an Arrhenius behavior with a factor of two increase in the activation energy compared to diffusion. This is attributed to the phase transformation process controlling the deuterium absorption/desorption kinetic behavior when the Pd–D system is within the miscibility gap.

SANS measurements of deuteride (hydride) formation in single crystal Pd

Abstract In situ small-angle neutron scattering (SANS) measurements of deuteride precipitation in well-annealed, undeformed single crystal Pd have been performed at room temperature to observe the morphology of the precipitate as the deuterium fraction was increased over a range of 0.005 to 0.097 [D]/[Pd]. Supporting measurements were made of the room temperature deuterium solubility isotherm in the single crystal material and of the progressive lattice mosaic broadening by gamma-ray diffraction analysis. The use of single crystal material eliminated the effect of grain boundaries on the precipitation process and allowed the orientation of the deuteride precipitates relative to the host Pd lattice to be established. The SANS data for deuterium loading beyond the solid solution phase are dominated by a d Σ /d Ω ∼ Q −2 scattering response well fit by a model cross section of small plate-like precipitates with a thickness of 25–30 A. This response showed little indication of a preferred habit plane and the plate thickness was independent of the total deuterium loading. The volume fraction of these small plates grew uniformly with increased deuterium loading, but was surprisingly small, reaching only 8×10 −5 at 0.060 [D]/[Pd]. This means that almost all of the deuterium resides in some other precipitate structure. The SANS data in the lowest Q region showed a second, anisotropic scattering component that can be interpreted as the Porod tail from plate-like particles of grossly larger dimensions. These plates favor a (001) α habit plane, normal to the elastically soft [001] α direction. Post-measurement visual inspection of the sample surfaces showed the presence of an oriented, quasi-periodic one-dimensional array of plates with thicknesses of approximately 0.1 to 0.2 mm and with an orientation in agreement with the anisotropy of the SANS data. We conclude that most of the deuteride phase precipitates in these large structures.

Effect of lattice defects and temperature transition rates on the deuteride (hydride) particle morphology and phase transformation thermal hysteresis in niobium

Small-angle neutron scattering (SANS) measurements have been performed to investigate deuteride particle morphology and the phase transformation temperature hysteresis in low-concentration Nb-D alloys. Deformation either by cold rolling and or by previous deuteride cycling induced a coarse deuteride particle distribution. This observation is attributed to a more heterogeneous precipitation process facilitated by the dislocation defects and/or dislocation substructure. Deuteride precipitation in the deformed samples was observed immediately upon crossing the incoherent solvus during temperature reduction, again consistent with dislocation-aided nucleation. Deuteride dissolution was observed at the very onset of heating for the cold-rolled material, an observation unique among the samples characterized here. This is attributed to the availability of elastic accommodation energy for deuteride particles embedded in the severely work-hardened host matrix. In other words, the elastic energy assists dissolution, consistent with a theoretical model developed by Puls (1984 Acta Metall. 32 1259-69). The effect of temperature reduction transition rates was also investigated. Rapid, direct cooling (at 2-3 K min-1) resulted in a much finer deuteride particle distribution - a factor of 200 increase in the particle number density and a factor of ten reduction in characteristic particle size compared to well annealed single crystal Nb. The thermal hysteresis was also affected by the temperature transition rates, with a significant reduction of the hysteresis for the slowest cooling rates. This implies that at least part of the recorded hysteresis in the well annealed material is dependent on the temperature transition rate.

Corrigendum to ‘SANS measurements of deuteride formation in single crystal Pd’: [J. Alloys Comp. 292 (1999) 134–147]

correct crystallographic orientation. This is the most We report here an error in the crystallographic orientaimportant figure with regard to our error because it shows tion of sample SC17 given in a previous publication [J. the orientation of the observed anisotropic Porod response Alloys Comp. 292 (1999) 134]. Two high-symmetry ̄ ̄ relative to the high-symmetry directions in the sample Q directions, [001] and [110], contained in the Q plane of the plane. The large deuteride plates responsible for this Porod SANS measurements of SC17 were inverted in Figs. 8 and ̄ response, originally placed on the (001) habit plane, are 9 of this publication. Fig. 9 is reproduced below with the a